The present invention relates to a drum-type motor, especially to a drum-type motor with inner gear, which has high efficiency and provides high torque output at low rotation speed
The conventional outer-rotor drum-type motor 51, as shown in FIG. 7, comprises a stator 511, an outer rotor magnet 512, a magnet shell 513, two side plates 514, a spindle 315 and a motor casing 415. In this conventional outer-rotor drum-type motor 51, the drum of the motor is directly driven by the outer rotor for operation. However, when the drum-type motor is operated in low rotation speed condition such as electrical bicycle, the motor output power of the motor is proportional to the motor rotation speed with reference to the formula:
T=P/N=Axc2x7Bxc2x7Lxc2x7D2xc2x76.1xc3x97108
Wherein
T: motor torque (N/cm)
N: motor rotation speed (rpm)
B: magnetic flux density
D: outer radius of rotor (cm)
P: output power (Watt)
A: current density (Amp/cm)
L: length of rotor (cm)
As can be seen from above formula, the power (P) of the motor is low when the motor is operated at low rotation speed and fixed outer rotor radius. To increase the motor output power at low rotor rotation speed, the current density (A) and the magnetic flux density (B) should be increased. However, this imposes a serious problem to the mechanical structure of the motor. To increase the current density (A), the thickness and turn number of the coils should be increased. However, the coils generally occupy a specific area ration on the winding groove of the rotor.
To overcome above problems, the present invention provides a drum-type motor combining planet gear and spindle motor. The planet gear is arranged on lateral side of the spindle motor and the axis of the spindle motor is functioned as sun gear. The drum-type motor of the present invention has an inner gear engaged with the planet gear. The inner gear and the sun gear have such tooth ratio that the drum-type motor of the present invention has high torque at low rotation speed.
The present invention is intended to provide a drum-type motor, which has high efficiency and provides high torque output at low rotation speed. The dissipated power of the motor is as follow:
W=I2R
W: dissipated power of the motor (Watt)
I: current (Amp)
R: copper resistance of coil (xcexa9)
As can be seen from above formula, the output power of the motor depends on the operation current (I) and the copper resistance (xcexa9) of coil. If the motor can provide sufficient torque at low operation current (I), the motor should have low copper resistance of coil and high operational efficiency. Therefore, the drum-type motor is designed to operate at low rotation speed and high torque with high rotation speed and high efficiency.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: